|Version 18 (modified by john@…, 9 years ago) (diff)|
on the wiki:
- The Control.Concurrency module
Papers and other docs:
- Concurrent Haskell (the original paper, including a semantics)
- Extending the Haskell FFI with Concurrency (a specification of the interaction between concurrency and the FFI, with a semantics)
- A Draft report addendum (a shorter version of the above paper).
- Software Transactional Memory
- Vital for some modern applications and large applications commonly require it.
- Stable MVar implementation is well understood and tested.
- Imposes non trivial implementation constraints.
- Providing a 'select' and non-blocking IO would be enough to allow people to implement something like it themselves in haskell and are provided by most systems as primitives.
- Things like the 'poor man's concurrency monad' can achieve some of the benefits
- Standardise on Concurrent Haskell without STM. It is our view that even in the presence of STM, MVars offer functionality that is distinct from STM and separately useful, so this leaves room for growth.
- Use the semantics from Extending the Haskell FFI with Concurrency
- Decide how much pre-emption is acceptable, and figure out how to specify this.
- Should we specify what an implementation that doesn't provide concurrency should do? (e.g. provide an implementation of MVars in terms of IORefs, so that concurrency-safe libraries can be written portably).
- Require bound thread support, or make it optional? (YHC has concurrency with non-blocking foreign calls, but doesn't have bound threads as yet.)
what is provided
At least the following interface will be provided
- Control.Concurrent.MVar - everything except addMVarFinalizer
- Control.Concurrent - ThreadId?,myThreadId,forkIO,yield,threadWaitRead,threadWaitWrite,threadDelay,threadSetPriority,threadGetPriority
the FFI must be able to handle 'concurrent nonrentrant' imports, but not necessarily 'concurrent reentrant' ones.
- if any haskell thread is runnable then at least one thread will be running.
- foreign calls marked 'concurrent' will not interfere will the above rule.
initial proposal is here:
Misc library stuff
yield is guarenteed to choose an alternate thread if another one exists and is runnable.
 priorities are advisory, but higher priority threads should be run in preference to lower priority ones.
threadDelay guarentees the thread will wait as long as its argument at a minimum. it may be blocked for longer.
 may be moved to another module, routines working on Handles should be added too.
 possibly, if we decide to go with priorities
Optional extensions to basic standard
These are optional extensions a compiler may implement. In some implementations they may entail a run-time cost to non-concurrent code or a compiler might need a special option to enable them. However, A compiler is not required to provide more than one concurrency model as long as it can meet the requirements of the standard and any options it claims to support.
If a compiler documents that it supports one of the following options, then it must adhere to the rules of that option as well.
Optional Feature 1 - Preemption
The standard only requires a progress guarentee, that a thread is always running, making progress. If an implementation supports context switching during arbitrary, perhaps even pure, computations and meets the stronger fairness guarentees, then it can be said to support the 'Preemption' option.
- no starvation
new library calls provided
- mergeIO, nmergeIO
Optional Feature 2 - OS threads
The implementation allows multiple haskell threads to run at once via the operating systems threading service. May take advantange of SMP architectures.
- bound threads
- concurrent reentrant supported